1. Field of the Invention
The present invention relates to recording media such as recording paper for ink-jet recording which have high ink (recording liquid) absorption ability and a moderate ink diffusing property and give a high picture element density (optical density of a picture element) as well as a good shape ink dot.
2. Description of the Prior Art
The ink-jet recording process is a recording method in which ink droplets are ejected through nozzles by an ink discharge technique such as electrostatic attraction caused by high voltage application, imparting mechanical vibration or displacement to the ink by means of piezo-electric elements, or some other technique, and are caused to adhere onto a recording medium. This recording method generates limited noise and permits high speed printing and multicolor printing.
For ink-jet recording, water-base inks are chiefly used in view of safety and aptitude for printing and generally comprise a colorant and a liquid medium.
Recording media hitherto used generally for ink-jet recording are sheets of common plain paper. However, improvements in the performance of ink-jet recorders, such as increased recording speed and the capability of multicolor recording, are accompanied by growing needs for ink-jet recording media to meet more demanding specifications.
The required characteristics are as follows: (1) Ink absorption must be as fast as possible. (2) An ink dot, when superposed upon a previously provided ink dot, does not breed thereinto. (3) The ink dots must have a precise and uniform diameter. (4) The ink dots have shapes close to a perfect circle and smooth outlines. (5) Recording media must have high brightness. (6) Colorants in inks applied must present good colors.
On the other hand, in ink-jet recording, original images are reproduced generally in a way such that the original image to be recorded is finely divided into equal sections (picture elements) and each of the picture elements is expressed with one ink dot or plural ink dots. In such a recording system, a sufficient picture element density (overall optical density of each picture element reproduced) is necessary in order to give a sufficient optical density to recorded images. For instance, when one picture element is expressed with one ink dot by the use of a definite amount of ink droplet of a definite colorant concentration, the ink droplet applied should spread as uniformly as possible over the entire area of the picture element. The reason for this is as follows: When the diameter of the ink dot fixed in a picture element is small, the overall density of the picture element is generally low, though the density of the ink dot itself is high; on the contrary, when the diameter of the ink dot fixed is large, the overall density of the picture element is high, though the density of the ink dot itself is low; and when the ink droplet spreads over the entire area of the picture element, the overall picture element density is maximized. This is because the overall picture element density depends mainly upon the inked area relative to the non-inked area.
On a recording medium having a higher ink diffusing property, recording can be carried out at a higher speed by increasing the area of each picture element.
Accordingly, it is desirable for ink-jet recording media to have, in addition to the above cited characteristics, such a moderate ink diffusing property that an ink droplet applied on a picture element having a prescribed area will spread uniformly over the entire area thereof.
For instance, when common wood-free paper is used for ink-jet recording, the ink absorption therein is bad, the diameter of resulting ink dots is not sufficiently large, the shape thereof is much inferior, and the desired high, uniform picture element density cannot be obtained. Commercial non-coated paper also gives unsatisfactory results though the ink-absorption therein is sufficient, that is, ink runs along fibers of the paper, the degree of ink diffusion varies from point to point on the paper, the shape and size of ink dots are difficult to control, and the density of ink dots varies locally, giving no uniform picture element density.